Thin wall bobbins and method for constructing same



Nov. 2, 1965 s. HORBACH 3,215,964

THI BBINS AND T Tiq.5.

TH E

Flu E mrogfw" United States Patent 3,215,964 THIN WALL ROBBINS AND METHOD FOR CONSTRUCTING SAME Stephen Horbach, 40 Glen Road, Mountain Lakes, NJ. Filed May 2, 1962, Ser. No. 191,900 7 Claims. (Cl. 336198) This invention relates in general to thin wall, high strength coil forms and to the method for constructing such structures. More particularly, this invention relates to a thin wall high strength bobbin usable in manufacturing electrical coils.

The coil forms and bobbins used in the electrical and electronic industries are frequently subject to considerable stress in use. The material out of which the bobbin is made must provide the high strength required to retainthe electrical coil windings, which consist of many turns of copper wire, without deforming or breaking off at the flanges due to the stress of the windings.

In addition the increased use of printed circuits require bobbins which incorporate terminal prongs so that the coil can be plugged into the printed board.

Further, the coil windings must be soldered to the terminals and, in many cases, the terminals must in turn be soldered a second time to be connected to the rest of the circuit in which the coil functions. Thus the coil form (bobbin) must Withstand a fairly high temperature without deforming and without weakening the terminal to bobbin mechanical connection.

The above problems can be met by prior art bobbins and coil forms only by using a thermosetting material with a thick wall design. Where resistance to high temperature is required and thus thermosetting materials must be used, it becomes necessary to design the core and flanges of the coil forms (bobbins) to be relatively thick (.080"-.150).

A further serious problem in the use of previously known thermosetting bobbins is that the attachment of terminals or solder lugs is a costly and slow process.

There is a great need in the electrical and electronic industries for smaller and lighter electronic parts. The demands of miniaturization additionally call for increased heat resistance of parts. Thin wall, high strength bobbins are known but only .When made out of a thermoplastic material such as nylon. These nylon bobbins withstand stress because of thin flexibility and toughness. However, nylon will flow and distort under the temperatures likely on soldering and thus prove unsatisfactory for bobbins with terminals.

The principal object of this invention is to provide a .molded, thin wall, high strength coil form.

It is a more specific object of this invention to provide bobbins and coil forms for use in electronic circuitry having thin wall, hgh'strength characteristics.

It is a further object of this invention to provide such structures with terminals such that the terminals can be soldered without weakening the bond between terminal and structure.

In brief, this invention involves molding of the desired thin wall structure from a glass fiber diallyl phthalate material. The structure molded from such material provides the proper combination of physical strength and heat resistance with minimum wall thickness and minimum weight. The structure so molded includes holes in which metal terminals may be permanently inserted. The holes are tapered at their outer end to facilitate the insertion of the terminals. The terminals are inserted by a three step process which involves, first, aligning the terminals with the holes, second, inserting the terminals partially in the holes under low pressure, and third, hammering the terminals in the holes under high pressure to achieve a pressed fit.

3,215,964 Patented Nov. 2, 1965 In this fashion the process overcomes the problem of the molding in of metal terminals during the molding stage. The terminals retain their fit after soldering because of the bobbin material used. The resulting bobbin with terminals resists both temperature and stress.

Other objects and features of this invention will be apparent from the following detailed description and diagrams, in which:

FIG. 1 is a perspective view of a completed bobbin with terminals;

FIG. 2 is a side view of the bobbin of FIG. 1;

FIG. 3 is a base view of the bobbin of FIG. 1;

FIG. 4 is a longitudinal cross section through one of the holes of the bobbin of FIG. 1 before a terminal has been inserted;

FIG. 5 is a longitudinal cross section view illustrating the first step in inserting the terminals into the molded bobbin;

FIG. 6 is a longitudinal cross section view illustrating the second step in inserting the terminals; and

FIG. 7 is a longitudinal cross section illustrating the last step in inserting the terminals.

FIGS. 1, 2 and 3 show the type of bobbin 10 which is particularly appropriate as an embodiment of this invention. The bobbin stem 11 provides the core around which the coiled wire is wound. The large number of turns of wire that are typically wound on the stem 11 causes considerable stress on the flange 12 and base 13. It is desirable that the flange 12 and base 13 be as small as possible in order to be the minimum size and minimum weight desired for parts in modern components. The base 13 must be thick enough to hold the terminals 14 and thus the flange 12 is the more crtical portion of the bobbin 10.

Prior art bobbins have been molded from thermosetting materials which materials require that the flange 12 be made at least 0.080 inch thick, and frequently 0.150 inch thick, in order to withstand the stress necessary to retain the electrical coil winding. Alternatively, prior art bobbins have provided relatively thin flanges, in the range of 0.015 inch thick where made of a flexible thermoplastic material such as polyamide (nylon). However, as has been pointed out, the thermoplastic materials flow and deform under the heat necessary to solder the coil windings to the terminals 14 and/or to solder the terminals 14 to the rest of a circuit.

The structure of this invention is a coil form (bobbin), the body and flanges of which are made of a thermosetting molding compound which has been reinforced with glass fiber. The thermosetting compound is necessary in order to provide the heat resistance required in applications where the terminals 14 are soldered. The glass fiber provides a reinforcement in the bobbin walls so that the stem 11 as well as the flange 12 can be made as thin as it is now possible to make such parts with thermoplastic materials. The bobbin 10 illustrated can be successfully made with a flange thickness of between 0.012-0.015 inch.

A glass fiber diallyl phthalate (DAP) has been found to be a preferred combination of thermosetting compound with glass fiber reinforcement for this application. The composition may be made by coating DAP directly on to glass fibers. There is nothing critical about the proportion of glass fiber used but it is preferable that the glass fiber be in the order of magnitude of 40% by weight of the composition. It is preferable that the glass fibers have a uniform orientation and thus run approximately parallel to one another. It is possible to use a composition with glass fibers having a random orientation but the strength of the bobbin walls made of such material is considerably less than the strength of the walls when made from a DAP reinforced with oriented glass fibers. It is also possible to use other thermosetting molded materials reinforced with glass fibers such as a glass fiber reinforced polyester. However, glass reinforced DAP has been found preferable because it permits molding bobbins with thinner walls than if other materials were used.

The Durez plastics division of the Hooker Chemical Corporation provides a molding compound which meets the preferred requirements. Their compound Number 18783 is an oriented glass fiber reinforced diallyl phthalate molding composition.

Many of the thermosetting compounds have been tried and found unsatisfactory. The addition of resins or granular glass to the thermosetting compound has resulted in too small an increase of strength to provide thin flanges which will withstand the pressure of winding.

In the bobbin illustrated, the base 13, as a practical matter, should have a minimum thickness of 0.070 inch so that the terminals 14 can be inserted to a'depth of 0.06 inch, which depth is necessary if the terminal is to have an adequate pull out strength. A three pound pull out test is normally applied to these terminals 14.

The shape and geometry of the hole 16 (see FIG. 4) into which the terminals 14 are pressed is critical if the terminals are to withstand the three pound pull out test. A minimum interference fit of 0.002 inch between the hole 16 and the terminals 14 is necessary in order to assure passing the three pound pull out test which is an electrical industry standard for such applications. Thus the diameter A in FIG. 4 must equal the terminal 14 diameter less 0.002 inch. To assure that the terminals 14 are properly aligned within the hole 16 and are not bent or deformed when press-fit into the hole 16, the hole 16 must have a tapered section near its opening. The tapered length B is preferably between 0.015 and 0.040 inch, while the outer end of the taper C is usually between 0.002 and 0.008 inch larger in diameter than the terminal hole diameter A.

The preferred process for making these bobbins 10 involves first separately molding the body of the bobbin 10 and then press-fitting the terminals 14 into the hole 16.

In molding, it is preferable to use a single gate for each bobbin in order to maintain maximum strength. An oriented glass fiber reinforced DAP composition is then cut into half inch long strips, the strips being approximately three-eighths of an inch wide by less than oneeighth of an inch thick. The required quantity of strips is then fed to the mold and the molding operation is carried out in the usual fashion.

When a completed bobbin has been molded, it is then placed in a fixture and held in position to receive the terminals 14. A set of terminals 14 are arranged in another fixture facing the base 13 of the bobbin. The terminals 14 are then brought up to the outer opening of the respective holes 16 in the bobbin base 13. Power is next applied to the fixture which holds the terminals 14 so that the terminals 14 may be press fit into the hole 16. However, the press fit is in two steps in order to avoid the bending of the terminals 14. A delay action may be incorporated into the power drive to assure that these two steps are carried out. During the first step the terminals 14 are inserted part way into the hole 16, generally a distance somewhat less than the taper length B so that each terminal is properly aligned within the hole. Then, and only then, is full pressure applied to press fit the terminal 14 into the body of the hole 16.

It is important that a space of between 0.005 and 0.008 inch be left at the bottom of the terminal hole 16 so that material which is scraped from the walls of the holes 16 during the press fit Will have a place to settle and thereby prevent bending of the terminals 14.

It should be remembered in connection with the above description that the terminals 14 though usually circular in cross section may have square or triangular cross sections. Other modifications of the embodiment described will be obvious to one skilled in this art and it is intended in the following claims to cover all such modifications.

I claim:

1. An article of manufacture comprising a molded glass fiber reinforced diallyl phthalate thin wall coil form, the bulk of said glass fibers being no greater than substantially one-half inch in length, said coil form having a stern adapted to serve as the core around which coil wire may be wound and at least one flange extending outwardly from one end of said stem to retain whatever wire is wound on said stem, said flange having a thickness substantially no greater than 0.040 inch.

2. An article of manufacture comprising a molded glass fiber reinforced diallyl phthalate thin wall coil form, the bulk of said glass fibers being no greater than substantially one-half inch in length, said glass fiber constituting substantially 40% by weight of said glass fiber reinforced diallyl phthalate, said coil form having a stern adapted to serve as the core around which coil wire may be wound and at least one flange extending outwardly from one end of said stem to retain whatever wire is wound on said stem, said flange having a thickness substantially between 0.012 and 0.020 inch.

3. An article of manufacture comprising:

a glass fiber reinforced diallyl phthalate molded coil form having a stem, a flange and a base, the bulk of said glass fibers being no greater than substantially one-half inch in length, said stem being adapted to serve as the core around which coil wire may be wound, said flange extending outwardly from one end of said stem to retain whatever wire is wound on said stern, said fiange having a thickness substantially no greater than 0.040 inch, said base being at the other end of said stem and including terminal holes, and

metal pin terminals extending from said holes, said terminals having an interference fit relationship with said holes in said base, said terminals having a substantially smooth surface in a longitudinal direction over the length of said terminals that is in said interference fit relationship with said holes.

4. An article of manufacture comprising:

a glass fiber reinforced diallyl phthalate molded coil form having a stem, a flange and a base, the bulk of said glass fibers being no greater than substantially one-half inch in length, said stem being adapted to serve as the core around which coil wire may be wound, said flange extending outwardly from one end of said stem to retain whatever wire is wound on said stern, said flange having a thickness substantially no greater than 0.040 inch, said base being at the other end of said stem and including terminal holes, and

metal pin terminals having a square cross-section extending from said holes of said base, said terminals having an interference fit relationship with said holes in said base, said terminals having a substantially smooth surface in a longitudinal direction over the length of said terminals that is in said interference fit relationship with said holes.

5. An article of manufacture comprising:

a molded glass fiber reinforced diallyl phthalate coil form having a stem, a flange and a base, the bulk of said glass fibers being no greater than substantially one-half inch in length, said stem being adapted to serve as the more around which coil wire may be wound, said flange extending outwardly from one end of said stem to retain whatever Wire is wound on said stem, said flange having a thickness substantially between 0.012 and 0.020 inch, said base having terminal holes and being located at the other end of said stem, said glass fiber constituting substantially 40% by weight of said glass fiber reinforced diallyl phthalate, and

metal pin terminals having a square cross-section extending from said holes of said base, said metal pin terminals having an interference fit relationship with 6 said holes of said base, said terminals having a subaligning a metal pin terminal having a square crossstantially smooth surface in a longitudinal direction section at one of said holes, over the lentgh of said terminals that is in said ininserting said terminal into the tapered portion of said terference fit relationship with said holes. hole, and 6. The method of manufacturing a thin wall coil form 5 press fitting said terminal into substantially the remp i ing the ps mainder of said one of said holes.

preparing strips of oriented glass fiber reinforced diallyl phthalate molding composition in lengths of sub- References Cited by the Examiner stantially no more than one-half inch in the direction UNITED STATES PATENTS of said oriented glass fibers, and 1 transfer molding a coil form body having a stern and 2,537,061 1/51 Kohnng 339221 X at least one flange extending outwardly from one 2,640,903 6/ 53 Kohfing 1 X end of said stem, whereby coil wire may be wound 2,803,043 3/ 57 Stephenson said stem and will be retained thereon by said ,712 8/58 Pollard et a1. 264--328 fiange, said flange having a thickness substantially 2,899,655 8/59 Forte et al. 336198 no greater than 0.040 inch. 2,947,959 8/ 60 Polzella et al. 336-198 7. The method of manufacturing a thin wall coil form 2,949, 41 0 Quackenbush g comprising the Steps Ofr 2,949,642 8/60 Lieberman 1s 59 preparing strips of oriented glass fiber reinforced diallyl 3,117,294 1/64 Muszynski et aL phthalate molding composition in lengths of substantially no more than V2 inch in the direction of OTHER REFERENCES said oriented glass fibers, transfer molding a coil form body having a stern, a CollForm (Du r m Fortune June 1945' flange and a base, said Stem being adapted to Serve Modern Plastics Encyclopedia issue for 1961, vol. 38, as the core around which coil wire may be Wound, said flange extending outwardly f one end f Plastics Properties Chart, Supplement to Modern said stem to retain whatever wire is wound on said Plastics Encyclopedia, issue September 1960, copyright stem, said flange having a thickness substantially 1960 y Plastics Catalogue p 575 Madiscn no greater than 0.040 inch, said base having terminal New York 22, NY.

holes and being located at the other end of said stem, said terminal holes being tapered at their JOHN BURNS, Primary Examinerouter portion, 

1. AN ARTICLE OF MANUFACTURE COMPRISING A MOLDED GLASS FIBER REINFORCED DIALLYL PHTHALATE THIN WALL COIL FORM, THE BULK OF SAID GLASS FIBERS BEING NO GREATER THAN SUBSTANTIALLY ONE-HALF INCH IN LENGTH, SAID COIL FORM HAVING A STEM ADAPTED TO SERVE AS THE CORE AROUND WHICH COIL 